This invention relates in general to dip coating and, more specifically, to a process for dip coating drums with a charge transport layer coating composition.
In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 in which a photosensitive member having at least two electrically operative layers is described. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain aromatic amine compound. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Other examples of homogeneous and binder charge generation layer are disclosed in U.S. Pat. No. 4,265,990. Additional examples of binder materials such as poly(hydroxyether) resins are taught in U.S. Pat. No. 4,439,507. The disclosures of the aforesaid U.S. Pat. No. 4,265,990 and U.S. Pat. No. 4,439,507 are incorporated herein in their entirety. Photosensitive members having at least two electrically operative layers as disclosed above in, for example, U.S. Pat. No. 4,265,990 provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely developed electroscopic marking particles. However, when the charge transport layer is applied by dip coating in extensively recirculated charge transport layer coating compositions, difficulties have been encountered due to the formation of coating non-uniformities such as axial or circumferential streaks appearing in the final charge transport layer. These streaks are undesirable because they may cause variations in the surface energy potential when an electrical charge is applied to the surface of the final charge transport layer which may cause printing defects in the final image, such as variations in light and dark final image print density. Also, stratification or segregation has been observed in the recirculated charge transport layer coating compositions which are believed to cause variations in viscosity control, coating thickness and electrical properties of the charge transport layer.
Variations in charge transport layer coating solution viscosity while coating, sudden and small charge transport layer coating solution flow rate changes, among other mechanisms, cause variations in coating material thickness. This thickness variation can be on any given drum or on different drums (batch-to-batch variations).
Thus, the characteristics of dip coating systems for forming a dip coated charge transport layer exhibit deficiencies which are undesirable for producing photoreceptors for high quality copiers, duplicators, printers, fax machines, multifunctional devices and the like.